The present invention relates to a method of producing hydrogen peroxide according to the cyclic anthraquinone process. The working solution to be used contains as the reaction carrier at least two differently substituted 2-alkylanthraquinones and/or the corresponding 2-alkyltetrahydroanthraquinones. In another aspect, the present invention relates to a novel reaction carrier.
In the so-called cyclic anthraquinone process for producing hydrogen peroxide, 2-alkylanthraquinones and/or their nuclear-hydrogenated 2-alkyl-xcex1- and/or xcex2-tetrahydroanthraquinones, functioning as reaction carriers, are hydrogenated in an organic solvent system in the presence of a hydrogenating catalyst with hydrogen or a gas containing hydrogen, whereby the reaction carriers are converted at least partially into the hydroquinone form. The solution, containing one or more reaction carriers in the hydrogenated or oxidized form and the organic solvent system, is generally designated as the working solution. After the hydrogenation stage, the working solution is freed from the hydrogenating catalyst and is then treated in the oxidation stage with an oxygen-containing gas, during which the quinone form of the reaction carriers re-forms together with the formation of hydrogen peroxide. After separation of the resulting hydrogen peroxide from the oxidized working solution, customarily done by extraction with water and/or with an aqueous solution containing hydrogen peroxide, the working solution is fed back to the hydrogenation stage. Aside from the cited stages, the process can also include a regeneration of the working solution, in which case anthraquinone derivatives such as anthraquinone epoxides formed in the cyclic process and which are inactive as reaction carriers are re-activated and/or 2-alkyltetrahydroanthraquinones are dehydrogenated to the corresponding 2-alkylanthraquinone derivatives and, as required, even losses of reaction carriers are replaced by the addition of the corresponding 2-substituted anthraquinones and/or their tetrahydro derivatives. A further stage relates to the regeneration of the catalyst in order to maintain a high activity. A survey of the cyclical anthraquinone process is contained in Ullmann""s Encyclopedia of Industrial Chemistry, 5th ed. (1989), vol. A13, 447-457 which is relied on and incorporated herein by reference.
High requirements are placed on the reaction carriers in order to assure the highest possible system output in large-scale plants with the lowest possible susceptibility to interruptions and the lowest possible loss of reaction carriers. One of the requirements concerns in particular the highest possible solubility of the reaction carriers in the solvent system both in the quinone form as well as in the hydroquinone form. The solubility of the hydroquinone form is decisive for the maximal H2O2 equivalent obtainable in constant operation (=g H2O2 per liter working solution). Further requirements concern the kinetics of hydrogenation and of oxidation; both reactions should take place as rapidly as possible. Since the hydrogenation and the oxidation are often influenced in an opposite manner by a change in the structure of a reaction carrier even a good reaction carrier system consisting of two or more components often represents only a compromise. Important too are the highest possible chemical stability of the reaction carrier in the catalytic hydrogenation, a high oxidation stability with respect to oxygen and hydrogen peroxide, and furthermore, a high stability with respect to acids and/or alkalis as are used in the regeneration. Finally, the reaction carrier should be as insoluble as possible in water, toxicologically harmless and economically available.
According to GB patent 1,252,822 one or more 2-alkylanthraquinones with 2 to 6 carbon atoms in the alkyl group, especially 2-ethyl-, 2-tert.-butyl- and 2-amylanthraquinone can be used in the anthraquinone process for producing hydrogen peroxide. The 2-alkyl-tetrahydroanthraquinones which form in the hydrogenation stage are also effective.
In the GB patent previously cited, not a single 2-alkylanthraquinone reaction carrier with 6 C atoms in the alkyl group is mentioned by way of example or even emphasized. In EP-A documents 0286610 and 0778085 2-hexenylanthraquinone is cited as a reaction carrier along with other 2-alkylanthraquinones and mixtures. Which of the possible hexenyl isomers is meant and whether or which advantages can be achieved therewith can not be gathered from these EP documents. It is known that as the chain length of the alkyl substituent increases in 2-alkylanthraquinones the quinone solubility increases; however, at the same time, and this is probably more important for practicability, the rate of hydrogenation drops off sharply. Thus, it was not obvious to seriously consider the use of a 2-C6-alkylanthraquinone as a reaction carrier.
It follows from JP-A 58 180452 and JP-A 59 051235 that 2-(4-methyl-3-pentenyl)-1,4-dihydroanthraquinone as well as 2-(4-methyl-3-pentenyl)anthraquinone obtainable therefrom and 2-(4-methylpentyl)anthraquinone can be used as reaction carriers for the production of hydrogen peroxide. The production of the cited compoundsxe2x80x94the parent compound is obtained by Diels-Alder reaction from 1,4-naphthoquinone and myrcenexe2x80x94can be gathered from these documents. Concerning the use of these compounds in the cyclical anthraquinone process for the production of hydrogen peroxide, it is solely mentioned that the same results can be obtained as with known 2-alkylanthraquinones.
The requirements placed on a good reaction carrier are occasionally only partly fulfilled when using a single 2-alkylanthraquinone and/or the corresponding 2-alkyltetrahydroanthraquinone, formed in situ, as a function of the operating conditions. Great efforts were therefore undertaken by those in this field of technology to improve the reaction carrier by using at least two different 2-alkylanthraquinones and/or their tetrahydro derivatives. However, advantages regarding the one or the other requirement on a good system of reaction carrier often oppose disadvantages regarding other criteria.
It is possible according to DE-AS 11 95 279 to raise the yield of hydrogen peroxide and/or to minimize the creation of byproducts during the hydrogenation if, instead of using a single 2-alkylanthraquinone such as 2-ethyl-, 2-isopropyl-, 2-sec.-butyl- or 2-tert.-butylanthraquinone, an almost eutectic mixture of at least two 2-alkylanthraquinones such as preferably 2-ethyl and 2.-sec.-butylanthraquinones is used in a weight ratio of 27 to 73 and if the degree of hydrogenation is held below 40%. A disadvantage of this method is the necessity of having to limit the degree of hydrogenation. An even more significant disadvantage is the unsatisfactory hydrogenation kinetics of these eutectic mixtures. Similar mixtures of two C1 to C4 alkylanthraquinones, which can be present in the so-called xe2x80x9canthraxe2x80x9d system but also the tetra system, are known from U.S. Pat. No. 2,966,397.
U.S. Pat. No. 4,374,820 suggests using a mixture of 2-tert.- butylanthraquinone and 2-sec.-amylanthraquinone including their tetrahydro compounds. This system does have good oxidation kinetics but unsatisfactory hydrogenation kinetics. On the other hand, DE-OS 11 12 051 and 11 06 737 recommend using a mixture of isomeric 2-amylanthraquinones, especially a mixture of 2.-sec.-amyl- and 2-tert.-amylanthraquinone and their tetrahydro derivatives as reaction carriers. A high H2O2 equivalent can be obtained with such systems on account of their good quinone and hydroquinone solubility; however, the unsatisfactory hydrogenation kinetics are also disadvantageous here, the consequence of which is a poor space-time yield.
Even the use of a reaction carrier system based on 2-ethylanthraquinone (EAQ) and 2-amylanthraquinone (AAQ) and their tetrahydro derivatives (THEAQ and THAAQ) is knownxe2x80x94see EP-A 0453949 and Chemical Economics Handbookxe2x80x94SRI International, June 1992, CEH Product Review Hydrogen Peroxide. A reaction carrier system on this basis (EAQ/THEAQ and AAQ/THAAQ) results in comparison to a reaction carrier system based on 2-ethylanthraquinone and 2-ethyl-tetrahydroanthraquinone in an increased H2O2 equivalent which can also be maintained under operating circulatory conditions. A disadvantage of the reaction carrier system based on EAQ/THEAQ and AAQ/THAAQ is its susceptibility to interruptions in the hydrogenation stage which makes itself noticeable in a reduced take-up of hydrogen. When using a suspension catalyst such as Pd-black this behavior forces the need for a relatively high circulation of hydrogenation catalyst and to further elevate it upon interruptions; however, this lowers the economy of the process.
It is therefore an object of the present invention to produce hydrogen peroxide using a working solution containing at least two differently substituted 2-alkylanthraquinones and/or their tetrahydro compounds which method exhibits to a lesser extent the disadvantages of the methods using the previously known 2-alkylanthraquinone combinations, especially those based on ethyl- and amylanthraquinone and their tetrahydro derivatives.
A further object of the system of the invention is to obtain, given good hydrogenation kinetics, a higher H2O2 equivalent which can be reliably controlled operationally and it should be less problem-prone.
The above and other objects of the present invention can be achieved by a method of producing hydrogen peroxide according to the cyclic anthraquinone process, comprising a hydrogenation stage, an oxidation stage and a stage for isolating the hydrogen peroxide using a working solution containing at least two differently substituted 2-alkylanthraquinones and/or their 2-alkyltetrahydro compounds.
A feature of the method of the present invention resides in that a working solution is used which contains (i) at least one reaction carrier selected from the group consisting of 2-(4-methyl-3-pentenyl) anthraquinone (IHEAQ), 2-(4-methylpentyl) anthraquinone (IHAQ) and their nuclear-hydrogenated di- and tetrahydroanthraquinone derivatives and (ii) at least one reaction carrier selected from the group consisting of 2-(C1 to C5) alkylanthraquinones and their tetrahydro anthraquinone derivatives, wherein reaction carriers according to (i) are present in an amount of 5 to 95 molar % relative to the sum of all reaction carriers.
The reaction carrier components according to (i) present in accordance with the invention are one or more compounds selected from the group consisting of 2-(4-methyl-3-pentenyl) anthraquinone, also designated subsequently as 2-isohexenylanthraquinone or abbreviated as IHEAQ, 2-(4-methylpentyl) anthraquinone, also designated subsequently as isohexylanthraquinone or abbreviated as IHAQ, 2-(4-methyl-3-pentenyl)-1,4-dihydroanthraquinone (=1,4-dihydro-IHEAQ), 1,2,3,4-tetrahydro-IHAQ (xcex1-THIHAQ), 5,6,7,8-tetrahydro-IHAQ (xcex2-THIHAQ), 5,6,7,8-tetrahydro-IHEAQ (xcex2-THIHEAQ) and intermediate stages of the hydrogenation of IHEAQ and IHAQ to THIHEAQ and/or THIHAQ under conditions of the anthraquinone process.
In the cyclic process primarily xcex2-THIHAQ along with a little xcex1-THIHAQ forms from IHAQ. The abbreviation THIHAQ stands for the isomeric mixture formed in the process.
In the method of the invention especially preferred reaction carriers according to (i) are IHEAQ and IHAQ as well as their xcex2-tetrahydro derivatives, especially xcex2-THIHAQ. As a result of the cyclic anthraquinone process IHAQ and THIHAQ form in the working solution when using IHEAQ as the component according to (i) after fairly long operation.
IHEAQ can be obtained by means of a Diels-Alder reaction from 1,4-naphthoquinone and myrcene and a subsequently introduced, base-catalyzed oxidation of the 1,4,4a,9a-tetrahydro-IHEAQ with air. IHAQ can be obtained by the hydrogenation of IHEAQ, e.g. on Pt/C. xcex1-THIHAQ can be obtained according to U.S. Pat. No. 1,425,250 by the hydrogenation of 1,4-dihydro-IHEAQ.
A further feature of the invention is also represented by 2-(methylpentyl)-5,6,7,8-tetrahydroanthraquinone (THIHAQ), a previously unknown reaction carrier for the cyclic anthraquinone process. This compound can be obtained by the hydrogenation of IHEAQ with Raney nickel or other hydrogenation catalysts such as Pt, Pd, Rh in metallic or carrier-bound form; it is also produced in the cyclic anthraquinone process from IHAQ and THIHEAQ. xcex2-THIHEAQ can also be obtained by a Diels-Alder reaction from tetrahydronaphthoquinone and myrcene with subsequent base-catalyzed oxidation.
The 2-alkylanthraquinones according to (ii) are one or more anthraquinones selected from the group consisting of 2-methyl-, 2-ethyl-, 2-n-propyl-, 2-iso-propyl-, 2-n-butyl-, 2-sec.-butyl- and 2-tert.-butyl- and 2-iso-sec.-amyl and 2-tert.-amylanthraquinone as well as 2-neopentylanthraquinone and/or their tetrahydro derivatives. The reaction carrier preferably contains 2-ethylanthraquinone (EAQ) and 2-ethyl-tetrahydroanthraquinone (xcex1- and xcex2-THEAQ) with xcex2-THEAQ predominating by far as a rule.
According to a preferred embodiment of the invention, the working solution contains as the reaction carrier essentially a combination of EAQ and IHAQ or IHEAQ with the corresponding tetrahydro compounds THEAQ and THIHAQ and/or THIHEAQ. The invention will be explained further using this system.
It is possible to increase a working solution containing essentially EAQ and THEAQ as reaction carriers with 2-isohexenylanthraquinone (IHEAQ) or isohexylanthraquinone (IHAQ) and/or their tetrahydro derivatives in order to raise the H2O2 capacity therewith. The molar amount of the sum of the anthraquinone- and tetrahydroanthraquinone derivatives with an isohexenyl- and/or isohexyl group, that is, the products according to (i), is usually between 5 and 95% relative to the sum of all active reaction carriers. During the phase of increasing, the molar amount of products according to (i) can also be below 5%. It is advantageous to adjust the molar amount of anthraquinone derivatives according to (i) to values in a range of 10 to 90%, preferably 20 to 80 molar %, and especially 20 to 50% and to then maintain it since the advantageous action of the combination in accordance with the invention, namely, a raising of the maximal H2O2 capacity with simultaneously better hydrogenation kinetics in comparison to the closest reaction carrier system, containing EAQ/THEAQ and AAQ/THAAQ is the clearest in this range.
After the addition of 2-isohexenylanthraquinone (IHEAQ) to the working solution the isohexenyl group is hydrogenated in the cyclic process to the isohexyl group. Although IHEAQ is not very oxidation-stable by itself (see example 3), surprisingly, no degradation of the isohexenyl group worthy of mention occurs during the oxidation stage in the cyclic anthraquinone process. The IHEAQ content slowly decreases during the cyclic process, whereas the content of IHAQ and of THIHAQ increases. THIHEAQ also formed at the start to a slight extent and drops back during the further course to values below the detection limit.
According to a preferred embodiment of the method of the invention the ratio of IHAQ to THIHAQ and of EAQ to THEAQ is maintained essentially constant during the cyclic process. To this end a part of the working solution is discharged out of the process and supplied to a known dehydrogenating regeneration stage, during which the tetrahydro derivatives present are dehydrogenated and the anthraquinone system is re-formed. The amount of working solution regenerated in this manner is fed back to the cyclic process. It is advantageous if 40 to 80 molar % of all reaction carriers are present in the tetrahydroanthraquinone form.
It was found that as the amount of reaction carriers of type (i) increases, that is, especially IHAQ, IHEAQ and THIHAQ, relative to the sum of all reaction carriers the hydroquinone solubility and therewith the maximal production capacity of hydrogen peroxide (g H2O2/l working solution) are raised. This capacity exceeds those analog systems containing the AAQ/THAAQ system instead of IHAQ/THIHAQxe2x80x94see examples 4.1 to 4.13. Furthermore, the capacity increases with an increasing amount of tetrahydroanthraquinones. Counter to the previous recognition according to which the hydrogenation kinetics become worse with increasing carbon number in the alkyl group of a reaction carrier the hydrogenation kinetics of the reaction carriers to be used in accordance with the invention according to (i) with an isohexyl- or isohexenyl substituent are surprisingly considerably better than the hydrogenation kinetics of isomeric 2-amylanthraquinones and 2-amyl-tetrahydroanthraquinones (AAQ/THAAQ)xe2x80x94see examples 5.1 to 5.6. The extraordinary advantage of the method of the invention is based on the fact that a higher H2O2 capacity with simultaneously improved hydrogenation kinetics is achieved with the reaction carrier combination in accordance with the invention in comparison to the closest previously known reaction carrier system (EAQ, AAQ and their tetrahydro derivatives).
The reaction carrier system of the invention can be used in any generic methods to produce hydrogen peroxide. In the hydrogenation stage known catalysts such as in particular those based on noble metals like Pd, Pt, Ir, Rh, Ru or mixtures of such noble metals, and Raney catalysts of Ni, Co or Fe can be used. The catalysts can be used as suspension catalystsxe2x80x94e.g. Pd-black or carrier-bound noble metalsxe2x80x94or in the form of fixed-bed catalysts. The carrier-suspension- and fixed-bed catalysts are in particular noble metals on an inorganic carrier such as SiO2, TiO2 zeolite, BaSO4, or polysiloxane. Finally, the catalyst can also be located on the surface of a monolithic ceramic carrier or of a honeycombed component with a sufficiently large surface. Common hydrogenation reactors are designed as loop reactors, fixed-bed reactors, mammoth pump reactors as well as reactors with integrated static mixers.
The hydrogenation is generally carried out at a temperature in a range of room temperature to 100xc2x0 C., especially 45 to 70xc2x0 C. The hydrogenation pressure is customarily in a range of approximately 100 kPa to 1 MPa, especially 200 kPa to 500 kPa. The hydrogenation is usually operated in such a manner that the hydrogen introduced into the hydrogenation circulation system is completely consumed and the degree of hydrogenation is maintained in a range of 30 to 80%.
The working solution containing the reaction carrier system in accordance with the invention generally contains two or more solvents in order to keep the reaction carrier components in the quinone form and the hydroquinone form in solution. Those solvents and solvent combinations can be considered which are known from previously known cyclic anthraquinone processes. Solvent combinations are especially suitable which contain, in addition to an aromatic benzine (multialkylated benzene), one or more solvents from the series of secondary alcohols such as diisobutylcarbinol, esters such as methylcyclohexylacetate, phosphoric-acid esters such as tris(2-ethylhexyl) phosphate, tri- and tetraalkylated ureas such as tetrabutyl urea, cyclic ureas, pyrrolidones, carbamates and N-alkylated caprolactams such as N-hexylcaprolactam.
Significant advantages of the method of the invention are: a capacity greater by at least 0.6 g H2O2 per liter working solution in comparison to the closest previously per liter; improved hydrogenation kinetics; a lesser susceptibility to disturbances in continuous operation; and a lesser amount of circulating palladium when using Pd-black as catalyst.